The problems associated with the introduction of ghosts in marine seismic exploration are well known. To date, almost all proposed solutions to this problem have been directed to filtering the seismic data acquired and manipulating the filtered data. Such data techniques have worked well on synthetic data, for ghosts introduced at the sea-surface and ocean bottom interfaces, but have been less successful in practice.
As described in Robertsson et al., U.S. Pat. No. 6,775,618, removing the ghost reflections from seismic data is for many experimental configurations equivalent to up/down wavefield separation of the recorded data. In such configurations, the down-going part of the wavefield represents the ghost and the up-going wavefield represents the desired signal. Exact filters for up/down separation of multi-component wavefield measurements in ocean bottom cable (OBC) configurations have been derived by Amundsen and Ikel, and are described in U.K. Patent Application Number 9800741.2. In fact, PGS has deployed a dual sensor component system for operation. See, e.g., U.S. Pat. No. 7,359,283 and U.S. Pat. No. 7,239,577. Apart from the difficulty with poles and zeros at critical wave numbers, they also require knowledge about the properties of the immediate sub-bottom locations as well as hydrophone/geophone calibration and coupling compensation. This drawback is even more problematic for downgoing signals from the air-sea interface.
A normal incidence approximation to the de-ghosting filters for data acquired at the sea floor was described by Barr, F. J. in U.S. Pat. No. 4,979,150. However, this technique is not as effective when the angle of incidence is away from vertical. Also, this technique does not completely correct for wide-angle scattering and the complex reflections from rough sea surfaces. Additionally, although known systems have used dual sensor streamers with a gimbaled velocity geophone and a group of hydrophones, such systems still suffer from a ghost effect on the source spectrum. In another application, a computer system and method for eliminating the effects of ghost reflections from marine seismic survey traces obtained via ocean bottom cables was suggested by Corrigan in U.S. Pat. No. 5,696,734.
Robertsson et al. thus suggested an improved de-ghosting method and system that utilized multi-component marine seismic data recorded in a fluid medium. The method made use of two types of data: pressure data that represented the pressure in the fluid medium, such as sea water, at a number of locations; and vertical particle motion data that represented the vertical particle motion of the acoustic energy propagating in the fluid medium at a number of locations within the same spatial area as the pressure data. The vertical particle motion data can be in various forms, for example, velocity, pressure gradient, displacement, or acceleration. A spatial filter separated up and down propagating acoustic energy over substantially the entire range of non-horizontal incidence angles in the fluid medium. The spatial filter was applied to either the vertical particle motion data or to the pressure data, and then combined with the other data to generate pressure data that has its up and down propagating components separated.
Unfortunately, this and other techniques in the art are based on the premise that the acquired data includes reflected noise which must be dealt with. In other words, techniques in the art attempt to remove the ghosts once they reach the receiver of the seismic system. After removing the effects of ghosting at the receiver locations, even with an effective filter, the effects of the ghost on the seismic source spectrum still remain. This reduces the amplitude of the seismic source spectrum at the low frequencies and in the vicinity of the ghost notches that occur at frequencies that are at integer multiples of v/2d, where v=acoustic velocity of the water and d=the depth of the source array. This adversely affects the signal to noise ratio at these frequencies. Norris, et al, in WO 2008/005001, state “Method and apparatus for producing a bubble curtain with a diversity of bubble diameters for purposes such as modifying the characteristics of a seismic source such as is used in seismic surveys.” It is understood that this bubble curtain has been deployed above a source array in order to attenuate the ghost effects at said source. In “The Marine Seismic Source” by G. E. Parkes and L. Hatton Fig. 4.6 p 75 they teach “The top far field measurement was made in good weather and the bottom measurement in poor weather. The reduced ghost size due to surface roughness is evident.” Therefore, there remains a need for a system and method of reducing the noise effects at the source, and the system disclosed herein is directed to filling this long felt need in the art.